Fastener using lubricated interference fit

ABSTRACT

The invention relates to a method for installing metallic fastener ( 10; 40 ) for the interference fit assembly of at least two structural elements ( 20, 22 ) comprising a through hole, the fastener comprising an enlarged head ( 12; 42 ), a shaft ( 14; 44 ) having an external diameter before installation that is greater than an internal diameter of the hole, said shaft comprising a conductive surface ( 26; 56 ). Before installation, at least the conductive surface ( 26; 56 ) is coated with a lubricating layer ( 30 ), which comprises a mixture of at least one polyolefin and one polytetrafluoroethylene, for example, having sufficient adherence to prevent its abrasion by manual manipulation of the fastener and being weak enough to be at least partly stripped from the conductive surface during the interference fit assembly of the fastener. The invention is applicable to the assembly of aircraft structures.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division of U.S. application Ser. No. 15/515,593 filed Mar.29, 2017, now U.S. Pat. No. 10,544,815, issued Jan. 28, 2020, which is aSection 371 of PCT/EP2015/072468, filed Sep. 29, 2015, which claimspriority to France Application Number 14 59269, filed Sep. 30, 2014, theentire contents of all of which are incorporated herein by reference.

The present invention relates to an electricity conductive fastener forapparatus of the aircraft type, to a method for obtaining such afastener and to a method for installing such a fastener in an assemblyof structural elements.

The new generation of airplanes comprise structures made of carbonfiber-reinforced composite material that have the advantage ofsignificantly reducing the weight of the airplane and which, contrary tometallic materials, are not subject to corrosion and to the fatiguephenomenon. However, contrary to a metallic structure, these compositematerials are poor electric conductors. They raise problems with respectto discharging currents and, consequently, to resistance to lightningstrikes. Therefore, any clearance that could exist between a fastenerand a hole in which the fastener is installed needs to be removed so asto create a close contact between the fastener and the structure. Thiscontact facilitates the passage of the current, prevents the creation ofsparks and prevents the presence of a volume of air, which, upon thepassage of strong currents, converts into a pressurized plasma that issusceptible to ignite fuel in the event that a fastener is installed ina tank structure. For this reason, the interference fit installation ofsleeved fastener devices has become widespread over recent years in thefield of structures made of composite materials, in order to allow thestrong electric currents generated by lightning to be discharged.

Sleeved fasteners are thus known from the prior art, from documents U.S.Pat. No. 8,382,413 and WO 2011/050040, that have a partial coating.These fasteners comprise a screw and a bushing installed with clearancein a hole. The bushing radially dilates in the structure when the shaftof the screw is inserted into the bushing, the shaft having an externaldiameter that is greater than the internal diameter of the bushing. Toallow the shafts of the screws to be inserted, these shafts compriseeither a lubricating varnish containing a pigment such as molybdenumdisulfide (MoS₂) or an organic coating containing aluminum pigments andlubricating pigments on an annular portion of their external surface.Both are reasonably considered to be electrically isolating due to thepresence of organic dielectric resin. The screws further comprise abare, electricity conductive annular shaft portion. The bushing is leftbare to allow the electric currents to pass from the screw to thecomposite structure.

The method for applying a partial lubricating coating on the shaftinvolves complex operations of masking screw portions per unit and doesnot allow bulk industrial treatment of large volumes of fasteners.Furthermore, the use of sleeved fastening systems is costly andincreases the weight of the airplane.

Threaded metallic fasteners are also known that are intended to beinserted directly into a non-sleeved structure by an interference fit.These fasteners can be treated in various ways: the fastener material iseither left bare after a passivation operation or is coated with purealuminum using the method called “IVD” or is coated with ananticorrosion coating that is phenol-formaldehyde resin-based comprisingaluminum pigments of the HI-KOTE™ 1 or HI-KOTE™ 1NC type marketed byHI-SHEAR Corporation, which is disclosed in the U.S. Pat. No. 3,979,351and EP 2406336, or is anodized in sulfuric acid or is coated withcadmium. The fasteners are all lubricated by a final layer of cetylalcohol, which is a lubricating grease, in order to allow a nut to beassembled on the threaded portion. The cetyl alcohol is easily removedduring a manual manipulation, vibrations during transportation or inautomated fitting systems or even during cleaning operations using acloth.

Aluminum exhibits very good electric conductivity and reduces thefriction in the threads. Nevertheless, its lubricating power is too weakto allow a fastener to be installed in a structure by interference fit.Indeed, an aluminum coated screw, even if it is coated with cetylalcohol, seizes very quickly when it is installed in a structure or abushing with an internal diameter that is less than the diameter of thescrew and may not be fully installed, especially when the thickness ofthe structure to be tightened is significant.

The aluminum pigment coating protects touching parts from galvaniccorrosion and prevents any seizing between the thread of the screw andthe tapping of a nut when screwing on said nut. It is not considered tobe an electricity conductor.

Sulfuric anodization is conventionally carried out on aluminum, titaniumor aluminum or titanium alloy parts in order to increase the corrosionresistance of the part. The layer has no lubricating power and isconductive when it is less than one micron thick.

None of these fastener variants can be mounted by interference fit in anarea of the airplane that is likely to be struck by lightning, eitherdue to insufficient lubrication of the shaft or due to excessiveelectrical resistance of the anticorrosion coating compromising thepassage of the lightning currents.

An object of the invention is to overcome these problems. Therefore, theobject of the invention is a metallic fastener for the interference fitassembly of at least two structural elements comprising a through hole,the fastener comprising an enlarged head, a shaft having an externaldiameter before installation that is greater than an internal diameterof the hole, said shaft comprising a conductive surface. Beforeinstallation, at least the conductive surface is coated with alubricating layer, the composition of which comprises a mixture of atleast one polyolefin and one polytetrafluoroethylene, comprising atleast 80% by weight of polyolefin and polytetrafluoroethylene, which isdetermined so as to have sufficient adherence to prevent its abrasion bymanual manipulation of the fastener and to be weak enough to be at leastpartly stripped from the conductive surface during the interference fitassembly of the fastener.

The lubricating layer has high lubricating power and low adherence onthe external surface of the shaft, yet which is sufficient so as not tobe removed by simple manual manipulation. When the metallic fastener isinserted into the structure by interference fit, this layer isprogressively and partly stripped from the shaft, leaving part of theexternal metallic surface of the shaft in contact with the structure,allowing the electric currents coming from the lightning to propagate inthe structure.

Of course, the invention is applicable to the assembly of mixedstructures, i.e. using composite and metallic materials, and to theassembly of purely metallic structures.

The fastener according to the invention can further comprise at leastone of the following features, taken individually or in combination:

-   -   the polyolefin is a polyethylene or a polypropylene;    -   the lubricating layer further comprises a desiccant and a        biocide;    -   the conductive surface comprises a sulfuric anodization layer        that is less than 1 micron thick;    -   the conductive surface is a metal having undergone a passivation        operation;    -   the conductive surface is an aluminum layer deposited in a        vacuum;    -   the conductive surface comprises the entire external surface of        the shaft;    -   the conductive surface comprises one portion of the external        surface of the shaft;    -   the conductive surface comprises an annular portion of the shaft        of the fastener;    -   the conductive surface comprises an axial portion of the shaft        of the fastener;    -   the lubricating layer further coats a threaded portion of the        fastener;    -   the fastener further comprises a traction tail and a shear        groove designed to break under a certain tensile stress

The invention further relates to a method for obtaining a metallicfastener for the interference fit assembly of at least two structuralelements comprising a through hole, the method comprising the steps of:

-   -   supplying a metallic fastener comprising an enlarged head, a        shaft having an external diameter before installation that is        greater than an internal diameter of the hole, said shaft        comprising a conductive surface;    -   spraying or soaking at least part of the fastener with/in an        alkaline solution comprising 60% to 80% by weight of an        aqueous-based solvent and 20% to 35% of a mixture of at least        one polyolefin and one polytetrafluoroethylene;    -   drying the fastener in order to obtain a dry, lubricating        polymer layer comprising a mixture of at least one polyolefin        and one polytetrafluoroethylene.

The method for manufacturing a fastener according to the invention canfurther comprise at least one of the following features, takenindividually or in combination:

-   -   the drying is carried out at a temperature between 60° C. and        80° C.;    -   a step of centrifugation is carried out after soaking and before        drying;    -   the step of spraying or soaking is preceded by a step of        depositing an aluminum layer or a step of sulfuric anodization.

The invention further relates to a method for installing a metallicfastener through a hole passing through at least two structuralelements, the fastener comprising an enlarged head, a shaft having anexternal diameter before installation that is greater than an internaldiameter of the hole, said shaft comprising a conductive surface. Beforeinstallation, at least the conductive surface is coated with a layer oflubricating polymers comprising a mixture of at least one polyolefin andone polytetrafluoroethylene. During the installation, during a step ofinserting the shaft of the screw into the structure through aninterference fit, the lubricating layer is partly stripped from theshaft of the screw.

The invention further relates to the use of at least one metallicfastener according to one embodiment of the invention, through a holepassing through at least two structural elements of an aircraft, inorder to dissipate electric currents in at least one structural elementwhen at least the head of the fastener is struck by lightning.

The invention and its various applications will be better understoodupon reading the following description and with reference to theaccompanying drawings, which are provided solely by way of anon-limiting example of the invention and in which:

FIG. 1 shows a fastener according to a first embodiment, beforeinstallation in a structure shown as a section view;

FIG. 2 shows a fastener according to a second embodiment of theinvention;

FIG. 3 shows a fastener according to a third embodiment of theinvention;

FIG. 4 shows the evolution of the friction coefficient of fasteners ofthe prior art with various diameters, coated with cetyl alcohol;

FIG. 5 shows the evolution of the friction coefficient of fastenersaccording to one embodiment of the invention, coated with lubricant;

FIG. 6 shows a fastener according to a fourth embodiment of theinvention.

Throughout these figures, identical elements use the same referencenumerals.

FIG. 1 schematically shows a side view of a fastener 10 beforeinstallation in a hole 20 made in structural elements 22, 24, which areshown as section views. Preferably, at least one structure is made of acomposite material, in this case the structure 24.

The metallic fastener 10 comprises a countersunk head 12, a cylindricalshaft 14 having an external diameter D₁ and a blocking portion 16 in theform of a thread having an external diameter D₂ that is measured at thecrest of the thread, with D₂ being less than D₁. A transition portion 18connects the cylindrical shaft 14 to the threaded portion 16. Thediameter D₃ of the hole 20 before installation of the fastener 10 isless than the external diameter D₁ of the shaft 14 and is greater thanthe diameter D₂ of the threaded portion, so that the threads are notdamaged when the fastener 10 is introduced into the hole 20.

In the example that is shown, the fastener 10 is a titanium alloy thathas undergone sulfuric anodization, creating an anodization layer thatis less than one micron thick over the entire external surface of thefastener. The external surface 28 of the threaded portion 16 and of thetransition portion 18 is coated with a second layer of lubricatinganticorrosion coating containing aluminum pigments of the HI-KOTE™ 1NCtype. This surface 28 is thus isolating (it is shown with a dottedbackground in FIGS. 1 to 3 and 6 ). The surface 26 of the head 12 and ofthe shaft 14 is electrically conductive.

In the example of FIG. 1 , the entire surface of the shaft 14 isconductive. However, only an annular portion (26A) of the shaft (FIG. 2) or one or more axial portions (26B) over the entire height of theshaft or over only part of the height of the shaft (FIG. 3 ) can beconductive. The other portions thus can be coated with a lubricatinganticorrosion coating containing aluminum pigments.

In the examples of FIGS. 1 to 3 and 6 , the fastener 10 is completelycoated with a lubricating layer 30, shown by a dot-and-dash line in thefigures. The thickness of the lubricating layer 30 can vary from 5 to 25microns, without this thickness affecting the mechanical performance orthe electric conduction of the fastener.

The applicant has surprisingly noted that such a screw 10 can beinstalled into a structure by interference fit and is able to conductthe electric currents when at least the external surface of the shaft iscoated with a lubricating layer 30 comprising a mixture of polyolefin(s)and polytetrafluoroethylene. The applicant has also noted that applyingthis same layer 30 on the lubricating anticorrosion coating of thethreaded portion significantly reduced the dispersion of the torqueneeded to install the fastener compared to the dispersion generatedthrough the use of cetyl alcohol. Furthermore, the lubricating power ofthis lubricating layer 30 is highly homogenous, regardless of thesubstrate on which it is applied.

The applicant thus has tested the interference fit installation of twobatches of fasteners 10 with different external diameters (diametersmeasured in 16^(th) of an inch), coated with different thicknesses of alubricating layer 30. The configurations tested and shown in table 1 areas follows:

-   -   Configuration 1: application of the lubricating layer 30 by        soaking and centrifugation;    -   Configuration 2: manual application of the lubricating layer 30        in one, one second spray gun pass;    -   Configuration 3: manual application of the lubricating layer 30        in one, 15 second spray gun pass.

The interference fit is measured by comparing the diameter D₃ of a holemade in an aluminum sample simulating a structure to the externaldiameter D₁ of the shaft 14 of the fastener 10 before installation. Thethickness of the lubricating layer 30 is measured before the fastener 10is inserted. The fastener 10 is installed in the hole by pushing on thehead 12, with the installation force being measured until the headtouches the sample. The sample is then cut in order to withdraw thefastener 10 without damaging the surface of the shaft 14, and thethickness of the lubricating layer 30 that remains after installation ismeasured. The average values of the results of these measurements areshown in table 1.

TABLE 1 Layer 30 Layer 30 Instal- thickness before thickness afterInter- lation installation installation ference force (μm) (μm) (mm) (N)Diam. Conf. 1 7.7 1.6 0.070  6 107 4 Conf. 2 8.3 1.7 0.068  6 730 Conf.3 23.5 2.1 0.099  6 099 Diam. Conf. 1 8.3 0.9 0.065 10 241 10 Conf. 29.8 1.0 0.067 11 294 Conf. 3 22.8 1.0 0.098 10 960

In all cases, the fasteners 10 were inserted by interference fit untilthe heads 12 came into contact with the surface of the test samples witha moderate installation force. Depending on the configurations,stripping of the lubricating layer 30 is observed, with the surplusremaining contained under the head or being pushed outwards. Thethickness of the lubricating layer 30 after insertion is highlyhomogenous and is between 0.9 and 2.1 μm.

A plurality of fasteners 10 installed by interference fit in samplesmade of a carbon fiber composite underwent lightning tests to verifytheir electrical conductivity. In order to complete these tests, thesamples comprising the fasteners are installed in a darkroom. Some heads12 of fasteners are struck by an extremely high amplitude current thatis equivalent to that of lightning. A camera records the test in orderto detect sparking or degassing around the fasteners 10. The samples arethen removed from the room and are cut in order to visually examine thesurface of the fasteners 10 with a scanning electron microscope.

The completed tests show that no sparking or degassing occurred duringthe test. The visual analyses show that no shaft 14 was damaged by thelightning, but that portions of the lubricating layer 30 and of theanodization layer were removed during the insertion. One theory is thatthe roughness of the carbon fibers in the composite material has anabrasive effect on a portion of the surface of the shaft, but that thiseffect does not continue over the entire height of the shaft.

According to one variant of the invention, the conductive surface 26 canbe made up of the bare or passivated surface of the metal forming thefastener or by an aluminum layer deposited in a vacuum on the baresurface of the metal forming the fastener.

Indeed, the applicant has surprisingly noted that the lubricating powerof the lubricating layer 30 did not depend on the support on which itwas applied. Therefore, the applicant has undertaken several insertiontests of fasteners 40 comprising a traction tail 50 (FIG. 6 ) with fourdifferent diameters, comprising various substrates coated either withcetyl alcohol or with a lubricating layer 30. The insertion was carriedout with an interference fit using a traction machine pulling thetraction tail and measuring the installation forces. A computationallows the friction coefficients to be determined between the fastener40 and the structure in which the fastener is inserted.

The tested configurations are as follows:

-   -   configuration 1 (HK1NC/AC/MIF): fastener coated with a first        layer of HI-KOTE™ 1NC lubricating anticorrosion coating and a        second layer of cetyl alcohol over the entire fastener;        installation by medium interference fit;    -   configuration 2 (HK1NC/organic lub./MIF): fastener coated with a        first layer of HI-KOTE™ 1NC anticorrosion lubricating coating        and a second lubricating layer 30 over the entire fastener;        installation by medium interference fit;    -   configuration 3 (HK1NC/AC/HIF): fastener coated with a HI-KOTE™        1NC lubricating anticorrosion coating and a second layer of        cetyl alcohol over the entire fastener; installation by high        interference fit;    -   configuration 4 (HK1NC/organic lub./HIF): fastener coated with a        HI-KOTE™ 1NC lubricating anticorrosion coating and a second        lubricating layer 30 over the entire fastener; installation by        high interference fit;    -   configuration 5 (OAS/HK1NC/AC/MIF): fastener comprising a first        sulfuric anodization layer, a second layer of HI-KOTE™ 1NC        lubricating anticorrosion coating on the threaded portion only        and a third layer of cetyl alcohol over the entire fastener;        installation by medium interference fit;    -   configuration 6 (OAS/HK1NC/organic lub./MIF): fastener        comprising a first sulfuric anodization layer, a second layer of        HI-KOTE™ 1NC lubricating anticorrosion coating on the threaded        portion only and a third lubricating layer 30 over the entire        fastener; installation by medium interference fit.

FIGS. 4 and 5 show the average results of the completed tests. FIG. 4shows the evolution of the friction coefficient of fasteners of theprior art with different diameters coated with cetyl alcohol. FIG. 5shows the evolution of the friction coefficient of fasteners 40according to one embodiment of the invention, which are geometricallyidentical to those used for the tests with cetyl alcohol and which haveundergone the same treatments, but are coated with a lubricating layer30 and not with cetyl alcohol.

The use of cetyl alcohol is clearly dependent on the substrate, as shownby the curve of configuration 5 in FIG. 4 , upwardly offset relative tothe curves of configurations 1 and 3, having the same substrate. Thecomputed friction coefficients thus vary between 0.045 and 2.10according to the diameter and the substrate.

By comparison, the use of the lubricating layer 30 shows that thefriction coefficient is within a narrow range that varies between 0.035and 0.07 regardless of the diameter and the substrate (FIG. 5 ).

The use of the lubricating layer 30 thus allows a fastener to beinstalled with a known torque or installation force in a very precisemanner that is even better than that which is obtained when using cetylalcohol. This layer 30 thus allows the risk of breakage of the fastenersduring installation to be limited, since the installation torque orforce, which is a function of the friction coefficient between thefastener and the structure, is only slightly dispersed.

The lubricating layer 30 is obtained, for example, from a liquidalkaline solution with a pH between 8 and 10, comprising at least onemixture of lubricating polymers dispersed in an aqueous base. In theliquid state, the composition of the solution is described in table 2.

TABLE 2 Compound % weight Solvent: Water 50-65 Solvent: Isopropanol10-15 Lubricating polymers 20-35 Other compounds 0-7

The solvent is a mixture of water and isopropanol, for example. As it isliquid at its operating temperature, the solvent dissolves and/ordilutes the other substances without chemically modifying them andwithout modifying itself. The isopropanol allows quicker drying of thesolution to be obtained once applied. The solvent evaporates upondrying.

The lubricating polymer is, for example, a composition comprising apolyolefin or a mixture of polyolefins, for example a mixture ofpolyethylene and/or polypropylene and polytetrafluoroethylene.

A polyolefin is an olefin polymer with a general structure of thefollowing type:—(CH₂—CRR′)_(n),in which the radicals R and R′ can be a hydrogen atom, a methyl (—CH₃),ethyl (—C₂H₅) or propyl (—C₃H₇) group.

The percentage by weight of polyolefin(s) in the solution is between19.5% and 30%, for example, and the percentage by weight of thepolytetrafluoroethylene in the solution is between 0.5% and 5%.

The solution can further comprise up to 4% by weight of a desiccant, forexample a silica gel. The desiccant allows the drying time of thesolution to be accelerated once applied.

The solution can further comprise up to 2% by weight of a surface-activeagent, such as polyethylene glycol ether. It improves the dispersion ofthe lubricating polymers in an aqueous-based solvent. The surface-activeagent evaporates upon drying.

The solution can further comprise less than 1% by weight of a biocidethat prevents the rapid formation of bacteria and of fungus likely todevelop in an aqueous-based solution.

The dry extract of such a solution comprises the lubricating polymersand, if it is contained in the liquid solution, the desiccant and/or thebiocide will be between 20% and 40% by weight. The dry extract comprisesat least 80% of lubricating polymers and at most 100%, if the solutioncomprises neither desiccant nor biocide.

Table 3 shows a preferred formulation of the solution.

TABLE 3 % weight Compound Solvent: Water 53.9 Solvent: Isopropanol 15Lubricating polymers 26 Including: PTFE 1 Other compounds Silica gel 3.0PEG Ether alcohol 2.0 Biocide 0.1

The dry extract of such a formulation comprises the lubricatingpolymers, the silica gel and the biocide, that is 29.1% by weight. Thelubricating polymers in this example constitute 89.34% by weight of thedry extract (26% of the 29.1% of dry material).

A mode for obtaining the fastener 10 comprising a lubricating layer 30is as follows.

In a first step, the metallic fastener 10 arriving from the forge and/orin a machined state is degreased, dried, then sandblasted, in order toprovide a clean external surface suitable for the adhesion of a coating.Optionally, a deposition of aluminum in a vacuum or anodic oxidation iscarried out partially or over the entire external surface of thefastener 10. If necessary, the threads of the fastener are coated with alubricating anticorrosion coating of the HI-KOTE™ 1 NC type.

In a second step, the fastener is loaded into a basket, then soaked inan alkaline solution that comprises 60% to 80% by weight of anaqueous-based solvent and 20% to 35% by weight of a mixture ofpolyolefin(s) and polytetrafluoroethylene dispersed in the solvent, forone minute at most.

The basket is centrifuged so as to spin-off the excess lubricant, thendried between 60° C. and 80° C. so that the solvents evaporate.

This method allows a layer 30 with a thickness of several microns to beobtained on the external surface of the screw 10 comprising at least 80%of lubricating polymers. It is to be noted that the drying temperatureis lower than the lowest melting temperature of the polyolefins. Indeed,a temperature higher than 150° C. causes yellowing and even browning ofthe polymer layer and contributes to a deterioration in the performanceof the coating.

When the entire surface of the fastener can be coated with thelubricating layer 30, such a method is advantageous as it allows largevolumes of fasteners to be treated at once.

A further mode for applying the lubricating layer 30 comprises using aspray gun to spray an alkaline solution comprising 60% to 80% by weightof an aqueous-based solvent and 20% to 35% of a mixture of polyolefin(s)and polytetrafluoroethylene over all or part of the fastener 10. In thiscase, the thickness of the lubricating layer 30 is controlled by theoperator and the centrifugation operation is not necessary.

Such a method is mainly used when only part of the fastener has to becoated. For example, if only one portion of the shaft has to be coated,the head and part of the shaft must be masked. In this case, thefasteners are installed in perforated plates that mask thenon-lubricated portion and the operator or a robot sprays the alkalinesolution only on the opposite side of the head.

The method for installing the screw 10 comprising a lubricating layer 30in a hole passing through at least two structures of an aircraft is asfollows.

The length of the fastener 10 is selected according to the thickness ofthe structures to be assembled, such that the threaded portion 16emerges from the structure 24 after having been inserted. The insertionof the fastener into the structure by interference fit is carried out bypushing on the head or by drawing the traction tail if the fastenercomprises a traction tail or even by screwing a nut onto the threadedportion in the case of fasteners with a conical shaft. By virtue of itshigh lubricating power, the lubricating layer 13 allows the screw to beinserted by interference fit. However, due to this same interference fitbetween the external surface of the shaft of the screw and the internalsurface of the structure, and due to the low adherence of the layer 30on the shaft of the screw, the lubricating layer 30 is partly strippedfrom the shaft of the screw both on its height and on its periphery,leaving conductive portions of the shaft 14 bare. The removal of thelayer 30 is not complete due to the manufacturing tolerances of thescrew and to the mode for applying the lubricant on the screw, whichvery locally can form pockets for storing lubricant that are severalmicrons thick.

The lubricating layer 30 sufficiently reduces the friction forcesbetween the screw and the structure to allow interference fitinstallation. Its adherence is strong enough to adhere to the shaft andto withstand any manual manipulation and it is weak enough to be partlyremoved during an interference fit assembly as the installationprogresses.

Once the head 12 of the screw 10 is in contact with the external surface32 of the structure 22, which is directed towards the outside of theaircraft, and the nut is in contact with the external surface 34 of thestructure 24, which is directed towards the inside of the aircraft, thefastener 10 is installed and assembles the structures 22 and 24 withoutclearance. When the head 12 of the screw 10 is struck by lightning, theelectric current is discharged into the structural elements 22, 24 or,in the case of composite structures, towards a metallic frame that isoptionally included on the surface 32 or inside the structural elementsvia the metallic portions of the shaft 14.

Of course, the invention is not limited to the embodiments describedabove. Thus, the shaft 14 of the screw can be conical and inserted intoa conical hole or into a sleeve, the internal surface of which isconical and the external surface of which is cylindrical, that isinstalled in a cylindrical hole. A fastener comprising such a sleeve isdisclosed in patent application FR 2946707 of the applicant. Theenlarged head of the screw can be of any known shape, for exampleprojecting or rounded.

By way of a variant, the fastener can comprise a traction tail that isprovided with traction grooves and is breakable at a shear groove, asshown in FIG. 6 . The fastener 40 comprises a projecting head 42, acylindrical shaft 44, a threaded portion 46, a shear groove 48, which isdesigned to break under a certain tensile or bending stress, and atraction tail 50 comprising traction grooves 52.

In this embodiment, the head 42 and the shaft 44 have an externalconductive surface 56 and the threaded portion has an external surface58 that is coated with an electrically isolating HI-KOTE™ 1NCanticorrosion coating. The entire fastener is coated with a lubricatinglayer 30 comprising a mixture of polyolefin(s) andpolytetrafluoroethylene, for example by soaking, centrifugation anddrying, as previously described. Of course, only the shaft 44 could becoated with the lubricating layer 30, by means of a spray gun and ofmasks, on only one portion that is of annular, axial or another shape.

This fastener 40 is installed by interference fit into a hole, thethickness of which is such that the traction tail 52 emerges from thestructure on the opposite side of the head 42. A tool comprising jawsgrips the traction grooves and pulls the fastener 40 into the hole. Thetraction tail is then broken at the shear groove 50 by an additionaltraction force or by bending the traction tail.

By way of a variant of all of the types of fastener described above, thethreaded blocking portion can be replaced by swaging grooves intended toreceive a swage collar instead of a screw nut.

The invention claimed is:
 1. A method for installing a metallic fastenera hole passing through at least two structural elements, the fastenercomprising an enlarged head and a shaft having an external diameter D₁before installation that is greater than an internal diameter D₃ of thehole, said shaft comprising a conductive surface, wherein at least theconductive surface includes a coating with a lubricating layercomprising a mixture of at least one polyolefin and onepolytetrafluoroethylene, and wherein during the installation, insertingthe fastener into the hole wherein the lubricating layer has acomposition determined so as to have sufficient adherence to prevent itsabrasion by manual manipulation of the fastener and to be weak enough tobe at least partly stripped from the conductive surface during theinterference fit insertion of the fastener and comprises at least 80% byweight of polyolefin and polytetrafluoroethylene, and during a step ofinserting the shaft of the fastener into the structure through aninterference fit, the lubricating layer is partly stripped from theshaft of the screw.
 2. The method according to claim 1, whereininserting the fastener includes inserting the fastener wherein thelubricating layer is, before installation, between 8.3 μm and 23.5 μmthick.
 3. The method according to claim 1, wherein inserting thefastener includes inserting the fastener wherein the lubricating layeris, after installation, between 0.9 μm and 2.1 μm thick.
 4. The methodaccording to claim 1, wherein inserting the fastener includes insertingthe fastener wherein the conductive surface comprises a sulfuricanodization layer that is less than 1 μm thick.
 5. The method accordingto claim 1, wherein inserting the fastener includes inserting thefastener wherein the conductive surface is a metal having undergone apassivation operation.
 6. The method according to claim 1, whereininserting the fastener includes inserting the fastener wherein theconductive surface is an aluminum layer deposited in a vacuum.
 7. Themethod according to claim 1, wherein inserting the fastener includesinserting the fastener wherein the conductive surface comprises theentire external surface of the shaft.
 8. The method according to claim1, wherein inserting the fastener includes inserting the fastenerwherein the lubricating layer further coats a threaded portion of thefastener.
 9. The method according to claim 1, wherein inserting thefastener includes inserting the fastener wherein the fastener furthercomprises a traction tail and a shear groove designed to break under acertain tensile stress.
 10. The method according to claim 1, whereininserting the fastener includes inserting the fastener wherein theconductive surface comprises only one portion of the external surface ofthe shaft.
 11. The method according to claim 10, wherein inserting thefastener includes inserting the fastener wherein the conductive surfacecomprises an annular portion of the shaft of the fastener.
 12. Themethod according to claim 10, wherein inserting the fastener includesinserting the fastener wherein the conductive surface comprises an axialportion of the shaft of the fastener.